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281 lines
14 KiB
XML
281 lines
14 KiB
XML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook MathML Module V1.1b1//EN"
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"http://www.oasis-open.org/docbook/xml/mathml/1.1CR1/dbmathml.dtd">
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<refentry id="glStencilOp">
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<refmeta>
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<refmetainfo>
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<copyright>
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<year>1991-2006</year>
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<holder>Silicon Graphics, Inc.</holder>
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</copyright>
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</refmetainfo>
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<refentrytitle>glStencilOp</refentrytitle>
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<manvolnum>3G</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>glStencilOp</refname>
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<refpurpose>set front and back stencil test actions</refpurpose>
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</refnamediv>
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<!-- eqn: ignoring delim $$ -->
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<refsynopsisdiv><title>C Specification</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>void <function>glStencilOp</function></funcdef>
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<paramdef>GLenum <parameter>sfail</parameter></paramdef>
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<paramdef>GLenum <parameter>dpfail</parameter></paramdef>
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<paramdef>GLenum <parameter>dppass</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1 id="parameters"><title>Parameters</title>
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<variablelist>
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<varlistentry>
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<term><parameter>sfail</parameter></term>
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<listitem>
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<para>
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Specifies the action to take when the stencil test fails.
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Eight symbolic constants are accepted:
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<constant>GL_KEEP</constant>,
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<constant>GL_ZERO</constant>,
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<constant>GL_REPLACE</constant>,
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<constant>GL_INCR</constant>,
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<constant>GL_INCR_WRAP</constant>,
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<constant>GL_DECR</constant>,
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<constant>GL_DECR_WRAP</constant>, and
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<constant>GL_INVERT</constant>. The initial value is <constant>GL_KEEP</constant>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>dpfail</parameter></term>
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<listitem>
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<para>
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Specifies the stencil action when the stencil test passes,
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but the depth test fails.
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<parameter>dpfail</parameter> accepts the same symbolic constants as <parameter>sfail</parameter>. The initial value
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is <constant>GL_KEEP</constant>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>dppass</parameter></term>
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<listitem>
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<para>
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Specifies the stencil action when both the stencil test and the depth
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test pass, or when the stencil test passes and either there is no
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depth buffer or depth testing is not enabled.
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<parameter>dppass</parameter> accepts the same symbolic constants as <parameter>sfail</parameter>. The initial value
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is <constant>GL_KEEP</constant>.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1 id="description"><title>Description</title>
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<para>
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Stenciling,
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like depth-buffering,
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enables and disables drawing on a per-pixel basis.
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You draw into the stencil planes using GL drawing primitives,
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then render geometry and images,
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using the stencil planes to mask out portions of the screen.
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Stenciling is typically used in multipass rendering algorithms
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to achieve special effects,
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such as decals,
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outlining,
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and constructive solid geometry rendering.
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</para>
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<para>
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The stencil test conditionally eliminates a pixel based on the outcome
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of a comparison between the value in the stencil buffer and a
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reference value. To enable and disable the test, call <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry>
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and <citerefentry><refentrytitle>glDisable</refentrytitle></citerefentry> with argument
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<constant>GL_STENCIL_TEST</constant>; to control it, call
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<citerefentry><refentrytitle>glStencilFunc</refentrytitle></citerefentry> or
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<citerefentry><refentrytitle>glStencilFuncSeparate</refentrytitle></citerefentry>.
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</para>
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<para>
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There can be two separate sets of <parameter>sfail</parameter>, <parameter>dpfail</parameter>, and
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<parameter>dppass</parameter> parameters; one affects back-facing polygons, and the other
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affects front-facing polygons as well as other non-polygon primitives.
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<citerefentry><refentrytitle>glStencilOp</refentrytitle></citerefentry> sets both front
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and back stencil state to the same values. Use <citerefentry><refentrytitle>glStencilOpSeparate</refentrytitle></citerefentry>
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to set front and back stencil state to different values.
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</para>
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<para>
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<function>glStencilOp</function> takes three arguments that indicate what happens
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to the stored stencil value while stenciling is enabled.
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If the stencil test fails,
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no change is made to the pixel's color or depth buffers,
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and <parameter>sfail</parameter> specifies what happens to the stencil buffer contents.
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The following eight actions are possible.
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</para>
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<variablelist>
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<varlistentry>
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<term><constant>GL_KEEP</constant></term>
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<listitem>
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<para>
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Keeps the current value.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_ZERO</constant></term>
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<listitem>
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<para>
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Sets the stencil buffer value to 0.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_REPLACE</constant></term>
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<listitem>
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<para>
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Sets the stencil buffer value to <emphasis>ref</emphasis>,
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as specified by <citerefentry><refentrytitle>glStencilFunc</refentrytitle></citerefentry>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_INCR</constant></term>
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<listitem>
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<para>
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Increments the current stencil buffer value.
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Clamps to the maximum representable unsigned value.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_INCR_WRAP</constant></term>
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<listitem>
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<para>
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Increments the current stencil buffer value.
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Wraps stencil buffer value to zero when incrementing the maximum
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representable unsigned value.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_DECR</constant></term>
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<listitem>
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<para>
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Decrements the current stencil buffer value.
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Clamps to 0.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_DECR_WRAP</constant></term>
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<listitem>
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<para>
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Decrements the current stencil buffer value.
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Wraps stencil buffer value to the maximum representable unsigned value when
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decrementing a stencil buffer value of zero.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_INVERT</constant></term>
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<listitem>
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<para>
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Bitwise inverts the current stencil buffer value.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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Stencil buffer values are treated as unsigned integers.
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When incremented and decremented,
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values are clamped to 0 and
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<inlineequation><mml:math>
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<!-- eqn: 2 sup n - 1:-->
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<mml:mrow>
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<mml:msup><mml:mn>2</mml:mn>
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<mml:mi mathvariant="italic">n</mml:mi>
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</mml:msup>
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<mml:mo>-</mml:mo>
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<mml:mn>1</mml:mn>
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</mml:mrow>
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</mml:math></inlineequation>,
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where
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<inlineequation><mml:math><mml:mi mathvariant="italic">n</mml:mi></mml:math></inlineequation>
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is the value returned by querying <constant>GL_STENCIL_BITS</constant>.
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</para>
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<para>
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The other two arguments to <function>glStencilOp</function> specify stencil buffer actions
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that depend on whether subsequent depth buffer tests succeed (<parameter>dppass</parameter>)
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or fail (<parameter>dpfail</parameter>) (see
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<citerefentry><refentrytitle>glDepthFunc</refentrytitle></citerefentry>).
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The actions are specified using the same eight symbolic constants as <parameter>sfail</parameter>.
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Note that <parameter>dpfail</parameter> is ignored when there is no depth buffer,
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or when the depth buffer is not enabled.
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In these cases, <parameter>sfail</parameter> and <parameter>dppass</parameter> specify stencil action when the
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stencil test fails and passes,
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respectively.
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</para>
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</refsect1>
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<refsect1 id="notes"><title>Notes</title>
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<para>
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<constant>GL_DECR_WRAP</constant> and <constant>GL_INCR_WRAP</constant> are available only if the GL
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version is 1.4 or greater.
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</para>
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<para>
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Initially the stencil test is disabled.
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If there is no stencil buffer,
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no stencil modification can occur
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and it is as if the stencil tests always pass,
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regardless of any call to <function>glStencilOp</function>.
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</para>
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<para>
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<citerefentry><refentrytitle>glStencilOp</refentrytitle></citerefentry> is the same as
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calling <citerefentry><refentrytitle>glStencilOpSeparate</refentrytitle></citerefentry>
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with <parameter>face</parameter> set to <constant>GL_FRONT_AND_BACK</constant>.
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</para>
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</refsect1>
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<refsect1 id="errors"><title>Errors</title>
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<para>
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<constant>GL_INVALID_ENUM</constant> is generated if <parameter>sfail</parameter>,
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<parameter>dpfail</parameter>, or <parameter>dppass</parameter> is any value other than the eight defined constant values.
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</para>
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<para>
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<constant>GL_INVALID_OPERATION</constant> is generated if <function>glStencilOp</function>
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is executed between the execution of <citerefentry><refentrytitle>glBegin</refentrytitle></citerefentry>
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and the corresponding execution of <citerefentry><refentrytitle>glEnd</refentrytitle></citerefentry>.
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</para>
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</refsect1>
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<refsect1 id="associatedgets"><title>Associated Gets</title>
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<para>
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<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument
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<constant>GL_STENCIL_FAIL</constant>, <constant>GL_STENCIL_PASS_DEPTH_PASS</constant>,
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<constant>GL_STENCIL_PASS_DEPTH_FAIL</constant>, <constant>GL_STENCIL_BACK_FAIL</constant>,
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<constant>GL_STENCIL_BACK_PASS_DEPTH_PASS</constant>, <constant>GL_STENCIL_BACK_PASS_DEPTH_FAIL</constant>,
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or <constant>GL_STENCIL_BITS</constant>
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</para>
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<para>
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<citerefentry><refentrytitle>glIsEnabled</refentrytitle></citerefentry> with argument <constant>GL_STENCIL_TEST</constant>
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</para>
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</refsect1>
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<refsect1 id="seealso"><title>See Also</title>
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<para>
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<citerefentry><refentrytitle>glAlphaFunc</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glBlendFunc</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glDepthFunc</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glLogicOp</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glStencilFunc</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glStencilFuncSeparate</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glStencilMask</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glStencilMaskSeparate</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glStencilOpSeparate</refentrytitle></citerefentry>
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</para>
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</refsect1>
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<refsect1 id="Copyright"><title>Copyright</title>
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<para>
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Copyright <trademark class="copyright"></trademark> 1991-2006
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Silicon Graphics, Inc. This document is licensed under the SGI
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Free Software B License. For details, see
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<ulink url="http://oss.sgi.com/projects/FreeB/">http://oss.sgi.com/projects/FreeB/</ulink>.
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</para>
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</refsect1>
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</refentry>
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